Phase 003: remove the coding-agent and ekko-agent runtimes

Every chat session now runs through the Hermes bridge, so the three chat
chains become feature-equivalent instead of three partially-maintained
dispatch paths. Keeping the coding-agent and in-house agent runtimes alive
meant every bridge change had to be mirrored into two more runtimes that
had already drifted.

Historical rows are read, not migrated: sessions stored with the retired
source normalize to cli on load, workflow graphs naming the retired agents
keep loading and run on the bridge, and the visible-source check plus
SOURCE_LABELS still recognize the old value so those sessions stay listed.

The public surface follows the code: the OpenAPI generator and the MCP
chat-run tool schema drop the six retired entry fields so external callers
are not told about runtimes that no longer exist.

harness-check gains a RETIRED_RUNTIME scan so the residual check is a
mechanical gate instead of a manual grep, with the two surviving read-side
compat sites recorded with reasons.

Refs: docs/phases/phase_003.md

tests/client/chat-input-draft.test.ts

@@ -154,11 +154,8 @@ describe('ChatInput draft persistence', () => {
     expect((wrapper.get('textarea').element as HTMLTextAreaElement).style.height).not.toBe('180px')
   })
 
-  it('shows context usage for coding-agent sessions', async () => {
-    const wrapper = mountForSession('session-codex', {
-      source: 'coding_agent',
-      agent: 'codex',
-      codingAgentId: 'codex',
+  it('shows context usage for the active session', async () => {
+    const wrapper = mountForSession('session-context', {
       inputTokens: 1200,
       outputTokens: 800,
       contextTokens: 2000,
@@ -170,12 +167,8 @@ describe('ChatInput draft persistence', () => {
     expect(wrapper.find('.context-bar').exists()).toBe(true)
   })
 
-  it('shows reasoning effort selector for coding-agent sessions', async () => {
-    const wrapper = mountForSession('session-codex', {
-      source: 'coding_agent',
-      agent: 'codex',
-      codingAgentId: 'codex',
-    })
+  it('shows the reasoning effort selector for the active session', async () => {
+    const wrapper = mountForSession('session-reasoning-effort')
     await nextTick()
 
     expect(wrapper.find('.n-popselect-stub').exists()).toBe(true)